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Nasa calls off shuttle's second launch try

Washington, December 9, 2007

Nasa canceled its second launch attempt of space shuttle Atlantis after a sensor in an emergency engine cutoff system failed again on Sunday, once more delaying Europe's major contribution to the International Space Station.

Atlantis and its seven astronauts -- five Americans, one German and one French -- are due to deliver Europe's Columbus science laboratory module to the space station.

The sensors in the shuttle's external fuel tank are part of a backup system to cut off the shuttle's main engines in case of an emergency during the climb to orbit. Two of the four sensors in the ship's hydrogen tank failed during Nasa's first launch attempt on Thursday.

"The ground rules that were laid out before we went into (Sunday's) launch attempt were that we would have to have four sensors that were mandatory for launch," launch commentator George Diller said.

"According to the plan that was laid out, the rest of the morning ... is probably going to evolve into a tanking test," Diller said, referring to a test of the fueling procedure.

Nasa did not immediately reschedule Atlantis' launch, although the next earliest opportunity is at 2:55 p.m. EST (1955 GMT) on Monday. Nasa has until Thursday or Friday for launch attempts this year.

The sensors, which operate like dipsticks to determine fuel levels, are part of a backup system to cut off the shuttle's three hydrogen-burning main engines if the tank runs dry due to a leak or other problem.

Running the engines without propellant could cause their pumps to break and possibly trigger a catastrophic explosion.

"It takes a great deal of courage for people to climb on board of that rocket ship and launch into space on any given day," shuttle program manager Wayne Hale said on Saturday.

Disasters in 2003 and 1986, which killed the crews aboard the shuttles Columbia and Challenger, have made NASA especially cautious about launch safety.

After's Thursday's scrubbed launch, managers met for two days and had decided to attempt the Atlantis launch again on Sunday despite the erratic sensors. The sensors have a history of glitches.

Engineers believed they had traced the problem to a subtle manufacturing issue and thought they solved it by replacing all suspect devices.

Hale told reporters late on Saturday that the continued problems show no one knows what is causing the discrepancies.

Nasa will start another round of investigations and engine upgrades to try to determine the cause of the problem, he added.

Nasa has 10 shuttle flights remaining to complete construction of the $100 billion space station. It also wants to make two resupply flights and a servicing call to the Hubble Space Telescope before the shuttle fleet is retired in 2010. - Reuters